Plasma probe and method for making same

ABSTRACT

A probe for measuring properties of plasma includes a shell, a contact extending through the shell and having a first connecting portion positioned in the shell, and a connector guide attached to a second connecting portion, the second connecting portion being detachably coupled to the first connecting portion. In another embodiment, a probe for measuring properties of plasma includes a shell, a contact extending through the shell, wiring extending from the contact and along an interior of the shell, and a coolant inlet line for injecting coolant into the interior of the shell for cooling the wiring. A method for cooling wiring positioned in an interior of a probe includes providing a coolant inlet line for injecting coolant into the interior of the probe and inserting the coolant inlet line in the interior of the probe such that the coolant cools the wiring. A method for assembling a probe having a shell and a contact extending through the shell and having a first connecting portion positioned in the shell includes attaching a connector guide to a second connecting portion adapted for detachably coupling to the first connecting portion and inserting the second connecting portion and the connector guide into the shell of the probe such that the second connecting portion becomes detachably coupled to the first connecting portion, the connector guide being for aligning the second connecting portion with the first connecting portion.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of Application No. 09/337,012, filedJun. 30, 1999, now U.S. Pat. No. 6,357,308, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to plasma probes, and more particularlyto the construction and assembly of Langmuir probes used to measureproperties of plasma, such as electron density and temperature.

FIG. 1 illustrates a conventional Langmuir probe. A typical Langmuirprobe 1 includes an elongated shell 2 with a closed end 3 and an openend 4 and is constructed of a dielectric, such as a ceramic material. Anelectrically conductive contact 5 extends through the closed end of theshell. The contact 5 forms a node 7 (shown here as a male pin connector)positioned within the shell 1. An exposed portion of the contact 5 maybe placed in direct contact with a plasma. A hermetic seal 6 may bepositioned in the interior of the shell towards the sealed end toprevent plasma from entering the interior of the shell. The contact 5extends through the hermetic seal 6, such that the tail end of thecontact forms the male pin connector. Some probes have electricallyconductive slugs 8 positioned around the contact and in the shellbetween the hermetic seal and the closed end of the shell to create adielectric effect. In such probes, the contact 5 may be divided intoforward and rearward sections 5 a, 5 b with the slug 8 providingelectrical conductance between the sections 5 a, 5 b.

A female pin connector 9 is detachably coupled to the node 7. Wiring Wruns from the female pin connector 9 and through the open end 4 of theLangmuir probe 1. The free end of the wiring W can be attached to ameasuring device which measures the potential created in the Langmuirprobe 1. The wiring W is usually a coaxial cable with a nonconductiveouter sheathing covering a braided wire shielding. The wiring W may bebiased with potential from a power source. An RF inductor filter R maybe coupled inline with the wiring W. The outer diameter of the filter Ris smaller than the inner diameter of the shell 2 but may have anexternal diameter close to the internal diameter of the shell 2.

A conductive ring 11 may be provided around the shell 2 near the closedend 3 of the shell 2 to serve as a reference electrode. An electricallyshielded grounding wire 13 is connected to the conductive ring 11. Also,an electrically conductive sleeve 17 may extend around the closed end 3of the shell 2. A Conflat® fitting 15 extends around the Langmuir probe1 towards the open end 4. The Conflat® fitting 15 seals against thecontainer holding the plasma.

Semiconductor fabrication equipment often use plasma processing.Exemplary processes in which plasma is used are dry-etching ofsemiconductors for microcircuits and plasma enhanced chemical vapordeposition (CVD). When performing semiconductor etching and deposition,it is best to have uniformity of the ion current density in the plasmareactor chamber. Such uniformity can be created by measuring the densitydistribution of the plasma during testing and making adjustments to theplasma reactor chamber and the operating conditions. During fabrication,the ion current density can be checked, and if required, adjustments toreach uniformity may be made.

Langmuir probes 1 can be used to measure properties of plasma, such aswhen conducting testing and diagnostics in the processes describedabove. The electron density and temperature of plasma can be derivedfrom Langmuir probe 1 measurements through the analysis of thecurrent-potential characteristics of the plasma. The contact 5 of theLangmuir probe 1 is a conductor, and when placed in direct contact withmoving charged particles found in the plasma, a current flows throughthe Langmuir probe 1. Based on the change in potential within the probe1, an estimation of the temperature and density of the electrons in theplasma can be made.

During the measurement of the properties of the plasma, the Langmuirprobe 1 heats up due to the current flowing through the wiring W and dueto the exposure of the probe 1 to the plasma. Probe 1 heating can leadto deterioration of the probe 1 both mechanically and with respect tothe RF filter R. Deterioration of the filter R can lead to total probefailure, or to a detuning of the filter R leading to high RF noise andresulting in inaccurate or misleading results. If the filter R becomesdamaged and inoperable from the heat, the only remedy is to replace thefilter R, which is a difficult and time consuming task.

Furthermore, the heat to which the probe 1 is exposed may cause thenonconductive shielding of the wiring W to melt, allowing core wires tocome in contact with braided wire shielding and to cause a short. Whenthis occurs, the wiring W must be replaced.

Yet another problem encountered with prior art Langmuir probes 1 is thatthe node 7 and female pin connector 9 becomes corroded due to the heat.Corrosion causes increased electrical resistance and must be removed foroptimum electrical connectivity. However, because the contact 5 is fixedby the hermetic seal 6, the node 7 is accessible only through theinterior of the shell 2. This makes removal of the oxidization verydifficult, as cleaning must be accomplished through the open end 4 ofthe shell 2. Thus, the wiring W must be removed before cleaning can takeplace.

To replace the wiring W, the old wiring W, filter R, and female pinconnector 9 are pulled from the shell 2 of the probe 1. New wiring,filter and female pin connector are then assembled. Generally, thewiring W is rigid enough to permit pushing of the female pin connector 9into engagement with the node 7 during reassembly. However, because ofthe flexibility of the wiring W and the dimensions of the shell 2, i.e.,very long with a small lumen, alignment of the female pin connector 9with the node 7 so that they may be reattached is very difficult. Thesize of the filter R generally prohibits insertion of tools to guide thefemale pin connector 9. Thus, a great deal of time may be spentattempting to reattach the node 7 and female pin connector 9. Further,once the probe 1 is reassembled, there is a strong probability that theinterior of the probe 1 will overheat and the shielding on the wire willmelt once again or the filter R will be damaged, again requiringdisassembly and reassembly.

Even still another problem with prior art Langmuir probes 1 is that thecurrent flowing through the wiring W fluctuates, creating RF “noise”. AnRF induction filter R helps remove some of the noise, but much of thenoise remains, which makes taking precise measurements difficult.Further, the filter R cannot be tuned to block different frequencies.Rather, the filter R must be removed and another filter that blocks thedesired frequencies installed.

SUMMARY OF THE INVENTION

The present invention is a plasma probe that is much more heat tolerantthan prior art Langmuir probes. Conduction and convection are utilizedto remove heat from the interior of the probe, thereby reducing theoccurrence of melted wiring and heat damage to filters. Also, thepresent invention assembles much more easily than known probes.Furthermore, the present invention uses capacitance to overcome thelimitations of the prior art with respect to the filtering of noise whentaking readings with the probe.

The present invention includes rigidly attaching the connector guide toa second connecting portion (e.g., a female pin guide) of a plasma probeto align the second connecting portion with a first connecting portion(e.g., a male pin guide). The guide has an outer diameter that is almostequal to but slightly smaller than the inner diameter of the shell. Asthe wiring is pushed into the shell, the guide slides along the interiorof the shell and guides the second connecting portion into attachmentwith the first connecting portion. Thus, rapid assembly and disassemblyare possible, permitting even routine maintenance to be performed morequickly than in the past.

As noted previously, the current flowing through the wiring fluctuates,making taking precise measurements difficult due to inductive effects.In an embodiment of the present invention, the guide is constructed ofan electrically conductive material to provide a capacitance between theguide and plasma outside the probe. That is, the guide forms one plateof a capacitor, the plasma forms another plate, and the shell acts as adielectric. In this way a large capacitance is created which filters thevariations in current to reduce the noise developed on the signaltraveling through the wiring of the probe.

To reduce the damage caused by heat in the interior of the shell, theguide may also act to cool the second connecting portion and the nearbywiring, thereby reducing the probability of meltdown of the componentsof the wiring and helping to prevent oxidation of the first and secondconnecting portions. In such case, the guide is preferably thermallyconductive to guide heat away from the second connecting portion. Toincrease the cooling effect of the guide even more, at least one coolingfin may extend from a rear face of the guide.

To enhance the cooling function of the guide, or used without the guide,a coolant inlet line may be inserted in the shell. The coolant inletline injects coolant (preferably air) into an interior of the shell toprovide convective cooling for the guide and wiring in the interior ofthe shell. The coolant may comprise air or another substance such asnitrogen gas. Optionally, a coolant outlet line may be inserted in theinterior of the shell to assist the escape of coolant from the interiorof the shell. For maximum cooling effect, an opening of the coolantoutlet line should be positioned towards an end of the shell oppositethe first connecting portion so that the length of the interior of theshell and the internal components therein are cooled. The coolant inletline may be coupled to the wiring before inserting the second connectingportion and the guide into the shell of the probe during assembly.

Before inserting the second connecting portion and the guide into theshell of the probe, the first connecting portion may be cleaned toremove any oxidation caused by overheating or simply from general use. Acleaning device that can be used has an elongate shaft with an open endand an abrasive inner lining positioned towards the open end of theshaft. The abrasive inner lining is rubbed on the first connectingportion for cleaning unwanted material from the first connectingportion.

An advantage of embodiments of the present invention is that theinternal components of the probe can be more easily removed and replacedthan was heretofore possible. This is particularly useful whenperforming periodic maintenance or replacing components.

Another advantage of embodiments of the present invention is that thecapacitance created by the guide reduces signal noise. This allowsmeasurements to be taken which are much more accurate than ever before.

Yet another advantage of embodiments of the present invention is thatthe internal components of the probe last much longer due to internalconvective cooling of the components. Furthermore, signal noise isreduced due to a lower operating temperature of the probe.

These and other advantages of the present invention will become apparentto those skilled in the art upon a reading of the following descriptionsof the invention and a study of the several figures of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a prior art Langmuir probe;

FIG. 2 is a cross sectional view of a partially assembled probe of thepresent invention without internal components;

FIG. 2a is a partial breakaway view of a cleaning device;

FIG. 3 is a cross sectional view of a probe illustrating cleaning of afirst connecting portion with the cleaning device of FIG. 2a;

FIG. 4 is a cross sectional view illustrating components being insertedin a probe;

FIG. 5 is a detailed view of the portion of the probe encircled at 5 inFIG. 4, illustrating in greater detail the connector guide and thesecond connecting portion;

FIG. 6 is a cross sectional view of an assembled probe of the presentinvention;

FIG. 7 is a cross sectional view of the probe along line 7—7 of FIG. 6;

FIG. 8 is a detailed view of a filter as encircled by circle 8 of FIG.6;

FIG. 8a shows the equivalent circuit of the tuning capacitor of FIG. 8;and

FIG. 9 is a cross sectional view of a new fully assembled probe in whichthe connector guide has heat dissipating fins.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 was discussed with reference to the prior art. In FIG. 2, anelongated tubular shell 12 of a probe 10 includes a closed end 14 and anopen end 16. The shell 12 is constructed of a dielectric, such as aceramic material, e.g., alumina. An exemplary length of the shell 12between its ends is less than about one meter with an outer diameter ODof the shell 12 being about ⅜ inch. An electrically conductive contact18 extends through the closed end 14 of the shell 12. The contact 18 hasa first connecting portion 20, shown here to be formed as a male pinconnector, positioned in the shell 12. The exposed portion 22 of thecontact 18 may be placed in direct contact with a plasma. Anelectrically conductive holding portion 21 may be positioned in theshell 12 towards the closed end 14 of the shell through which thecontact 18 extends. Optionally, the contact 18 may be divided intoforward and rearward sections corresponding to the first connectingportion and the exposed portion with the holding portion 21 providingelectrical conductance between the sections of the contact 18. Thispermits removal of the exposed portion 22 of the contact 18 forreplacement in the event it becomes damaged. Also optionally, anelectrically conductive sleeve 23 may extend around the exterior of theclosed end 14 of the shell 12. The sleeve 23 acts as a passive filter.Preferably, both the holding portion 21 and the sleeve 23 are made ofmetal.

A hermetic seal 24, also of a dielectric such as a ceramic, may bepositioned in the interior of the shell 12 towards the sealed end toinhibit plasma from entering the interior of the shell 12. The contact18 extends through the hermetic seal 24.

A conductive ring 26 coated in a dielectric material such as a ceramicmaterial extends at least partially around the shell 12 near the closedend 14 of the shell 12. An electrically shielded grounding wire 28extends from the conductive ring 26. A Conflat® fitting 30 extendsaround the shell towards the open end 16. The Conflat® fitting 30 can beused for mounting or sealing the probe to a container holding plasma.For example, the Conflat® fitting 30 may be attached to a bellows thatextends and retracts as the probe 10 is moved in and out of contact withthe plasma.

Before assembling the probe 10, the first connecting portion 20 ispreferably cleaned to remove any oxidation caused by overheating or fromgeneral use. A cleaning device 32 that may be used is shown in FIG. 2a.The cleaning device 32 has an elongate shaft 34 with an open end 36 andan abrasive inner lining 38 positioned towards the open end 36 of theshaft 34. To construct such a cleaning device 32, a braided wire core ofa coax cable may be deformed to place the braided wires in disorder andthen pushed about one half inch into the open end 36 of the shaft 34until flush with the open end 36 of the shaft 34, for example. Thebraided wire core may be secured to the shaft 34, such as by solderingto form the abrasive inner lining 38. A hole approximately the size ofthe first connecting portion 20 is poked in the braided wire.

Referring to FIG. 3, the cleaning device 32 is inserted in the interiorof the shell 12 until the first connecting portion 20 is inserted in theopen end 36 of the shaft 34. The abrasive inner lining 38 engages thefirst connecting portion 20 for cleaning unwanted material from thefirst connecting portion 20. The shaft 34 may be rotated as indicated atR and moved on and off of the first connecting portion 20 by translatingthe device 32 as is indicated at T to scour the first connecting portion20 with the abrasive inner lining 38.

FIG. 4 shows a second connecting portion 40 that is adapted todetachably couple to the first connecting portion 20 to make amechanical and electrical connection herewith. An exemplary secondconnecting portion 40 is a female pin connector. The second connectingportion 40 is connected to wiring 42. The wiring 42 runs from the secondconnecting portion 40 and through the open end 16 of the shell 12 of theprobe 10. The free end of the wiring 42 can be attached to a measuringdevice (not shown) which measures the potential created in the wiring42. The wiring 42 is preferably a coaxial cable with a nonconductiveouter sheathing covering a braided wire shielding. The braided wireshielding is separated from at least one inner wire by a layer ofinsulation. The wiring 42 may be biased with potential from a powersource 44. A filter 46 may be coupled to the wiring 42. Such a filter 46could be an RF induction filter for reducing RF noise in a signalpassing through the wiring 42.

A connector guide 48 is rigidly attached to the second connectingportion 40 to align the second connecting portion 40 with the firstconnecting portion 20. The wiring 42 can be coupled to the front 50 andback 52 of the connector guide 48 if the connector guide 48 iselectrically conductive, or the wiring 42 may extend through theconnector guide 48.

The connector guide 48 may be attached to the second connecting portion40 in any suitable manner, including threaded engagement, welding, pressfitting, soldering, swaging, etc. Preferably, the connector guide 48 isconnected directly to the second connecting portion 40 to maximize heattransfer between the connector guide 48 and the second connectingportion 40. However, if it is desired to electrically isolate theconnector guide 48 and the second connecting portion 40 while stillpermitting some thermal conduction between the connector guide 48 andthe second connecting portion 40, heatshrink tubing 54 may be placedaround the second connecting portion 40 and heat applied to shrink theheatshrink tubing 54 to the connecting portion 40. See FIG. 5. Theconnector guide 48 is then screwed and/or pushed onto the secondconnecting portion 40 such that threads of a threaded bore 56 of theconnector guide 48 engage the heatshrink tubing 54.

The connector guide 48 is preferably shaped like the interior of theshell and has an outer diameter that is almost equal to but slightlysmaller than the inner diameter of the shell 12 so that the connectorguide 48 is guided by the interior surfaces of the shell 12. Thus theconnector guide 48 preferably holds the second connecting portion 40centrally with respect to the interior of the shell 12. As the wiring 42is pushed into the shell 12, the connector guide 48 slides along theinterior of the shell 12 and guides the second connecting portion 40into attachment with the first connecting portion 20.

The connector guide 48 may be constructed of an electrically conductivematerial, such as a metal like aluminum or copper, for producing acapacitance between the connector guide 48 and plasma outside the probe10 with the shell 12 acting as the dielectric. In this way a largecapacitance is created to reduce noise on the signal probe.Alternatively, the connector guide 48 may be constructed of a dielectricsuch as a ceramic material for producing a smaller capacitance betweenthe wiring 42 extending through the connector guide 48 and plasmaoutside the probe 10. Optionally, the connector guide 48 may comprise anonconductive material, such as a plastic or alumina, if no capacitanceis desired. If the connector guide 48 is constructed of a nonconductivematerial, it may have limited RF filtering and heat dissipationbenefits.

To reduce the damage caused by heat in the interior of the shell 12, theconnector guide 48 preferably is used to cool the second connectingportion 40 and the nearby wiring 42, thereby reducing the chance ofmeltdown of the components of the wiring 42 and helping to preventoxidation of the first and second connecting portion 40 s. In such case,the connector guide 48 is preferably constructed of a thermallyconductive material to serve as a heat sink, which dissipates heat byconduction along the shell and by convection with gasses in the shell.To aid in convection, at least one cooling fin 58 may extend from a rearface of the connector guide 48 (see FIG. 9). There, convection carriesheat from the guide 48 to the air in the interior of the shell 12.

FIG. 6 illustrates a convective cooling system for a probe of thepresent invention. The cooling system includes a coolant inlet line 60inserted in the shell 12 which injects coolant into an interior of theshell 12 to enhance the cooling function of the connector guide 48 byconvectively cooling the connector guide 48, filter 46, and wiring 42 inthe interior of the shell 12. When used in an embodiment without theconnector guide 48, the coolant inlet line 60 cools the secondconnecting portion 40, wiring 42, and the filter 46. The coolant maycomprise air or another substance such as nitrogen gas. The pressure ofthe coolant in the coolant inlet line 60 can be adjusted, such as with avalve (not shown), to adjust the rate of flow of the coolant, therebyadjusting the rate of cooling. For example, a pressure of 80 psi withair as the coolant may be used to produce good cooling results in ashell 12 of the aforementioned dimensions.

Optionally, a portion of the coolant inlet line 60 may be coupled to aportion of the wiring 42, such as by binding or adhering them togetheras with tape or adhesive, before inserting the second connecting portion40 and the connector guide 48 into the shell 12 of the probe duringassembly. This ensures proper positioning of the outlet 62 of thecoolant inlet line 60 with respect to the wiring 42. Further, couplingthe coolant inlet line 60 to the wiring 42 adds support to the wiring42, reducing damage to the wiring 42 during assembly and disassembly ofthe probe 10 and allowing the use of thinner wiring.

Also optionally, a coolant outlet line 64 may be inserted in theinterior of the shell 12 for assisting the escape of coolant from theinterior of the shell 12. The coolant outlet line 64 reduces pressurebuildup in the interior of the shell 12, and may be necessary if theopen end 16 of the shell 12 needs to be sealed shut. For maximum coolingeffect, an opening 66 of the coolant outlet line 64 should be positionedtowards an end of the shell 12 opposite the first connecting portion 20so that a large portion of the length of the interior of the shell 12and the internal components found therein are cooled. For example, witha shell 12 having a length of about one meter, the coolant outlet line64 should extend about eight to ten inches into the shell 12 from theopen end 16 of the shell 12.

FIG. 7 illustrates the positioning of the coolant inlet line 60, coolantoutlet line 64, and wiring 42 with respect to each other in the interiorof the shell 12. For a shell 12 having an outer diameter OD of ⅜ inch,thin walled air tubes having an inner diameter 61 of ⅛ inch may be used.As shown, some coolant is allowed to escape from the shell 12 throughinterstices I between the coolant lines 60,64 and wiring 42.

In FIG. 8, a filter 46 is shown including a tuning capacitor 68. Thetuning capacitor 68 creates a small amount of capacitance that allowstuning of a frequency of a signal passing through the wiring 42. Tocreate the tuning capacitor 68, a first wire 70 is coupled to the wiring42 on one side of the filter 46. A second wire 72 is coupled to thewiring 42 on the other side of the filter 46. The first and second wires70,72 are twisted together to produce a small capacitance, e.g., a fewpicofarads. By adjusting a number of twists of the first and secondwires 70,72, the LC constant of the filter 46 may be adjusted.

Each of the wires 70,72 is shielded and the ends 74 of the wires are notelectrically connected. To allow precision tuning, the material of whichthe shielding is constructed may be changed. For example, teflon,plastic, and silicon shielding each yield a different capacitance.

FIG. 9 illustrates a fully assembled probe 10 of the present invention.In use, the closed end 14 of the probe is inserted in plasma such thatthe first connecting portion 20 is in contact with the plasma to createa current flow through the probe 10. Based on the change in potentialwithin the probe 1, an estimation of the temperature and density of theelectrons in the plasma can be made. The number of twists of the firstand second wires 70,72 can be changed to adjust the LC constant of thefilter 46.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andapparatuses of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

What is claimed is:
 1. A probe for measuring properties of plasmacomprising: a shell; a contact extending through said shell; a connectorguide having an outer diameter substantially equal to an inner diameterof said shell and rigidly holding an inner contact adapted for engagingsaid contact; wiring extending from said contact and along an interiorof said shell; and a coolant line for injecting coolant into saidinterior of said shell for cooling said wiring.
 2. A probe for measuringproperties of plasma as recited in claim 1 wherein said connector guideis thermally conductive, and wherein said coolant convectively removesheat from said connector guide.
 3. A probe for measuring properties ofplasma as recited in claim 1 wherein said connector guide comprises anelectrically conductive material for producing a capacitance.